This invention relates to improved processes and methods for electrodialysis of salts to produce acids and bases.
For general information, reference is made to my earlier U.S. Applications xe2x80x9cELECTRODIALYSIS APPARATUS,xe2x80x9d Ser. No. 08/784,050, filed Jan. 17, 1997 and xe2x80x9cAPPARATUS AND PROCESS FOR ELECTRODIALYSIS OF SALTS,xe2x80x9d Ser. No. 08/787,899 filed Jan. 23, 1997.
The invention relates to a use or addition of certain organic acids, particularly polyaminoacetic acids, in order to enhance the retention in a solution of divalent cations in the pH range of 2 or higher. The stoichiometric complexes, called xe2x80x9cchelates,xe2x80x9d formed with the multivalent cations, are effectively retained in a solution by an ion exchange membrane so that they are not transported out of a given process loop. The primary benefit of this procedure is that the divalent metals are kept in the solution within a given loop of the electrodialysis cell and, more particularly, in the vicinity of bipolar membranes used in cells, as described in my earlier applications. This retention, in turn, has a remarkably beneficial effect on the cell""s operation in terms of improving the process"" reliability, reducing power consumption and avoiding heating/melting problems.
Nanofiltration, chelating resin ion exchange, or other pre-treatments may be used in conjunction with the electrodialysis step to reduce the divalent and multivalent metals content in a salt feed stream or a base product stream. Within the electrodialysis cell itself, monovalent selective cation membranes may be used to further reduce the amount of divalent metals transported out of the feed loop and thereby reduce the amount of a chelating agent needed to retain the divalent metals in the transported product (usually higher pH) solution.
The invention enables essentially the same process to be used in many different systems, without requiring an extensive modification thereof. More particularly, the inventive process may be used in these and a number of other applications such as (a) the production of acids, organic or inorganic, in conjunction with a weak base such as ammonia. In this context, the invention is particularly well suited to a recovery of lactic acid from a fermentation derived ammonium lactate, especially a recovery in a two compartment cell, (b) recovery of sulfur dioxide from waste gases using a two compartment cation or anion cell, and (c) purification of ammonium salts for use in a fluid catalytic cracking (xe2x80x9cFCCxe2x80x9d) catalyst production using a two compartment anion cell, and (d) production of acids and bases in a three compartment cell.
Salts are byproducts or intermediate products of a number of chemical processes. Regenerable flue gas desulfurization processes use a sodium alkali to absorb the SO2, thus resulting in a soluble bisulfite salt, NaHSO3. Fermentation processes for organic acids (such as acetic and lactic acids) go through the intermediate production of salts, such as ammonium acetate or lactate. For example, the manufacturing of rayon/regenerated cellulose results in a generation of significant quantities of byproduct sodium sulfate.
Electrodialysis (xe2x80x9cEDxe2x80x9d) may be used to convert these and other soluble salts directly into their acid and base components. For example, in the case of organic salts, such a procedure will enable a direct recovery of the organic acid in a relatively pure form, while the co-product base (ammonia for example) may be recovered for reuse in the fermentation process in order to make pH adjustment. Thus, there is an economical and environmentally superior option for producing organic acids. In other instances, such as with sodium bisulfite or sodium sulfate, electrodialysis offers an environmentally superior route for recovering and/or recycling the acid, base components
Electrodialysis uses direct current as a means for causing a movement of ions in a solution. Electrodialysis processes are carried out in a stack arrangement comprising a plurality of flat sheet ion exchange membranes. To produce acids and bases from their salts, the process unit requires a means for splitting water. Two useful means for splitting water into hydrogen (H+) and hydroxyl(OHxe2x88x92) ions are:
(i) A bipolar membrane or a bipolar module including a combination of cation and anion membranes functioning as a bipolar membrane. Suitable bipolar membranes are available from Aqualytics, a division of Graver Water and Tokuyama Corporation.
(ii) An electrode set comprising an anode and a cathode. The electrodes, (particularly the anodes), are suitably coated for chemical stability, for minimizing power consumption, and for the formation of byproducts other than hydrogen (at cathode) and oxygen( at the anode). Suitable electrodes are available from Eltech Corporation, Electrode Products Inc., and others. One can also use a hydrogen depolarized anode to generate the H+ions in the aqueous solution and next to the anode.
When using a stack of bipolar membranes, the stack contains electrodes (anode and cathode) at either end of a series of membranes and gaskets which are open in their central area in order to form a multiplicity of compartments separated by the membranes. Usually, a separate cleaning solution is supplied to end compartments which contain the electrodes with special membranes placed next to the electrodes to prevent a mixing of the process streams with the electrode cleaning streams.
The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between adjacent membranes. The repeating unit is called a xe2x80x9cunit cellxe2x80x9d or simply a xe2x80x9ccellxe2x80x9d. A solution is usually supplied to the compartments either by internal manifolds formed as part of the gaskets and membranes or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another stack in order to optimize process efficiency. The change in the composition of a stream after one pass through the stack may be relatively small.
The solutions can be recycled by being pumped to and from recycle tanks. An addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
A known treatment of aqueous salt streams by electrodialysis (or electrolysis) forms an acid or a base from the salt. In order for a bipolar membrane, to function as a water splitter, the component ion exchange layers must be arranged so that the anion selective layer of each membrane is closer than the cation selective layer to the anode. A direct current is passed through the membranes in this configuration to cause water splitting with OHxe2x88x92 ions being produced on the anode side and a corresponding number of H+ ions being produced on the cathode side of the membranes. The dissociated salt anions move toward the anode and the dissociated salt cations move toward the cathode.
The electrolysis process works in a similar manner, with the water splitting occurring at the two electrodes. When a direct current is passed, water molecules are converted to oxygen gas at the anode along with an introduction of Hxe2x88x92 ions into the aqueous solution. At the cathode, the water molecules are converted to hydrogen gas along with the introduction of OH ions into the aqueous solution. In the hydrogen depolarized anode based electrolysis unit, OHxe2x88x92 ions are released into the aqueous solution next to the cathode, while the released hydrogen gas is forwarded to the catalytic hydrogen depolarized anode for H+ ion generation.
Electrodialysis equipment for acid and base production has three compartment cells comprising bipolar, cation and anion membranes, two compartment cells containing bipolar and cation membranes, a multichamber two compartment electrodialysis cells comprising bipolar and two or more cation membranes (or two or more anion membranes). A number of processes uses such equipment.
In keeping with an aspect of the invention, I have found that operating an electrodialysis cell in the presence of organic compounds binds or chelates with the multivalent metals to form a metal chelate buffer that dramatically enhances the performance of electrodialysis cells. Among other things, this binding or chelating reduces power consumption, produces a stable cell operation, and avoids a fouling of the membranes. These benefits translate into a significantly improved membrane life, improved process reliability, and reduced operating costs for the processes.
The chelating agent binds with the multivalent metal ions (calcium, magnesium, iron, etc.). When a chelating agent is added to feed solutions containing multivalent cations, the chelating agent strongly binds with the cations, forming larger size complexes. The ion exchange membranes retain these complexes within the compartment containing the feed solution. The multivalent cation transport across the cation exchange membranes is substantially inhibited. As a result, the fouling of the cation membranes is reduced or eliminated. Concurrently, the precipitation of the metals transported to the base loop is substantially abated.
When added to the base product solution, the chelating agent binds with the multivalent cations that may be present in the feed stream or transported from an adjoining loop, and mitigates the precipitation of the multivalent cations either on the bipolar membranes or within the base compartment, thus allowing the water splitting process to occur in a trouble-free manner over extended operating periods.